Solar power. It's a great idea. I like the idea of being independent and free of the grid. I like that (once installed) it uses a free source of energy. I like that it's clean, quiet, and odor free. And I do not regret at all installing a solar system on my house.
But it does have a few issues. And these are some things you should know and think about if you are planning to live off grid either in your tiny house or otherwise. I knew pretty much nothing about solar power and living off grid when I drew up the plans for my house, but since then, while still no expert, I have learned a lot of things. Some of these may be pretty obvious, others, for me at least, were not. I am not an electrician so feel free to laugh at some of my descriptions or word choices. I'm just trying to write all this out in a way that makes some sense now to someone who didn't know anything about the subject a few months ago.
First, think about what are your particular reasons for wanting to go off grid? To save money on utilities? Depending on the expense of the system you choose, how long you expect to use it/stay in one place, and the cost of power through your local utility, this may or may not add up to savings over time. To save the earth and be more green? Great, but think about the power used to produce each part of your solar system and what will happen to all those parts when they eventually wear out. Batteries for instance are very large, expensive, and there is no real good way to dispose of them once they reach the end of their life. You local power plant may actually be greener in the long run. To become more self reliant and free of dependance on or regulation by others? Probably nothing else will replace this reason very well. Because you live somewhere that has no access to the grid and you still want the (major) convenience of using many things powered by electricity? Not much else will solve this one either. The last two and primarily that very last one are the main reasons I am off grid. But they are all things to think about depending on what motivates you.
Then consider the other possible ways to have power and be off grid. You could also produce power via a gas generator, windmill, or water powered turbine. These each have their own pros and cons as well. Gas generators require gas and the time and expense involved to obtain it and are noisy and smelly. But they can be relatively cheep to purchase initially and easy to set up and move. Windmills require some serious construction and obviously a steady supply of wind in their particular location. They however then are pretty quiet and require no consistent operating costs. A turbine needs a steady supply of water that doesn't dry up, freeze up, or get clogged up with sediment. But if you have that, they run around the clock and are also free of ongoing fuel costs. Depending on your location and what's available, you may want to think about one or more of these options.
So now you've decided for whatever reason or combination of reasons that you do indeed want a solar installation. First you need to figure out how much power you actually use. So you need to know how much electricity every single item uses, as well as about how much of the time that item is actually turned on. This needs to include everything that takes any electricity at all. The obvious things tend to be air conditioning, fridges/freezers, lights, heaters, water pumps, cooking (depending on what method you use), etc. Less obvious things can include the heater or stove that is propane and still requires electricity to ignite, run a fan, power an internal clock and such. Or the "ghost" draw from things like a TV that may use a little power all the time, even when the power is turned off. Calculating up this list can be a bit complicated. More on that later.
Unless you want to install a large and very expensive system, you might want to consider how important high power use items like a cable TV box, toaster, coffee maker, vacuum, curling iron, clothes dryer, microwave, blender, hair dryer, etc are to you and possibly forgo them completely. Not that you can't have those things, you just need to plan for items like that pushing the power requirements way up and decide if each one is worth it to you. Some of those items use a lot of power the whole time they are on, but they are not on very long. I.E. a microwave or hair dryer. Others use much less power at any one time, but run a lot, such as a fridge or freezer. Either situation becomes a large power draw.
Do you want to run everything in your house on AC power or DC power? Or some combination of both. The best I understand the way this works is that one is a steady stream of current coming through your lines (DC) like a faucet that is turned on. The other is an intermittent pulse of power (AC), more like a lawn sprinkler. Either one is very capable of powering things, but they are totally different and the item being used has to be designed for the power type you are using. If you plug something designed for one thing into the other kind of power you will destroy it. (Maybe explosions, sparks, smoke, melting wires, and other general fun included.) The basic advantages of DC are that it requires less work to turn the power you have created via your solar panels into this, and it's a bit more efficient. It's also more dangerous (which is why everything in your on the grid house uses AC instead), there are many less cool things that are set up to use DC power, and they are often more expensive than their equivalent set up for AC. AC has basically all the opposites of those. It takes more work to turn the power you are generating into AC, and is less efficient. But it is safer, most things are designed to work with it, and they are often less expensive than a comparable item designed for DC power.
Solar power works when the sun is shining, not at night, not (or very little) when it's raining, snowing, cloudy, etc. Every day seems to include a night, and many days, depending on your location also include one of those other weather conditions. Also it helps if the sun hits your solar panels directly. When the sun in low in the sky, (all winter if you live very far north like me) behind a tree, or otherwise indirect or partially blocked, the power output is greatly reduced. All these things need to be considered when trying to figure out how much power you can actually produce.
Next, to have power all the other hours of the day when the sun is not shining, you'll need a battery bank of some kind to store the power you've generated. And you will have to have a charge controler to safely add the power coming in from your solar panels to your battery bank. When calculating how much power things you want to use will need, be aware that your power in your battery bank is going to be measured in DC amps. Almost everything you use is going to be powered in AC amps. And a rough calculation is to multiply the number of AC amps something uses by 11 (plus a little, it's 11.04 really) to find the number of DC amps you'd need to power it.
Next, pay attention to how many amps the solar panels you are looking at can produce. My 4 - 100 watt panels can put out, in perfect conditions like a totally clear day with the sun directly overhead and no shade or dirt on the panels, 5.29 amps each per hour. That means that total, again in perfect conditions, my set up can produce 21.16 amps of DC power per hour. That's about 2 AC amps per hour. Not very much. And like mentioned above, there are very few hours in most days where you will have these perfect conditions, if at all. These calculations are rough and ignoring details like line drop (the power you loose as electricity travels through a wire), loss while converting from one kind of power to another, and loss during storage. But they should help you get a general idea without making the calculations too complicated.
So I have a generator as a backup to my solar system. This does require buying gas. And having a charger that takes the power the generator produces and safely stores it in the batteries. My charger can add at the max, 25 amps of DC power per hour to my battery bank. So right now I'm running it every other day for at least 5ish hours to top off my batteries and make up the extra power I need beyond the ability of the weather and my solar panels. It's a very efficient and small generator, running almost 10 hours on less than a gallon of gas so this isn't too expensive. Especially with the current low gas prices.
My battery bank currently consists of 5 - 155 amp hour batteries wires in parallel. Meaning they are all connected to basically act as a single big battery. That gives me the ability to store 775 amp hours of DC energy. Which gives me just over 68 amp hours of AC power stored. Again not very much. And batteries like this don't last very long at all if you run them down to 0%. In fact, they last longest if kept around 80% or above. So try to size your battery bank so you'll never use more than 20% of it in any one day whatever your power needs are. I use somewhere between 10-15% of mine each day. Also, remember that to turn the power coming out of your battery bank into the AC power you will probably want to use, you'll need an inverter as well.
The amount of power I have stored would usually last me around 6 days if I ran my batteries from 100% to 0%. The items I choose to have that require electricity are as follows, roughly in order of the amount of power they use. Refrigerator/freezer (small and efficient), water pump, lights (all LED), internet router, laptop and phone charging, ignition in my propane oven, bathroom and hood vet fans, ignition and fan running of my now secondary propane heater, epilator, and a kitchen aid mixer which is used only rarely.
So as you can see, I've kept my electrical needs fairly low and my system is working very well for me. And right now, my system is coming in around just under $4000 total. With all the labor having been supplied by a very good friend or myself so not counted into that figure. When I have time, I want to write out a detailed accounting of this as well as more details on each piece of my particular system. Hopefully for now though, that all makes some sense to you if you are learning about installing a solar system. And maybe raises some things you haven't thought about yet. Let me know if you have any questions!